I have a problem for which i am not able to figure out a solution. I have 6 sheets (csv files). Each have a coloumn named "employee id". I want to find out common employees across these 6 files. Can anybody help me out with this? Thanks in advance.

Drag all the fields to make a unique record onto Rows, make them discrete, then put Number of Records on Columns. If you only see an axis from 0 to 2, then all the bars will be at 1 and all the records are unique, otherwise there are duplicate records. You can then use the one-click sort on the Number of Records axis to sort for the duplicates.

Could you also try this as well

Create a calculated Field

COUNT([Name]) - COUNTD([Name]) // Duplicates Only

and put it into Filters/Color Shelf

Or Follow the below post, here also I tried to answer similar question

One manner of doing this is to union the files together. When doing so tableau will create a column that identifies which csv they came from. When you then drag your employee ID onto the rows shelf you can then drag that new field name in as well. This will build a table that lists each table that each employee went into.